All About Wet Bulb Globe Temperature

What is WBGT?

Wet Bulb Globe Temperature is a measure of environmental heat stress. It is the gold standard for measuring how the human body experiences heat.

Four conditions shape the number

Temperature

Warmer air adds heat to the environment.

Humidity

Moist air slows the evaporation of sweat.

Sunlight

Sun and nearby surfaces add radiant heat.

Wind

Moving air can help sweat evaporate.

How to use WBGT

WBGT describes the heat environment. A policy for the activity turns that reading into decisions about timing, workload, rest, equipment, and when to stop.

Start with conditions at the activity site.

Use a WBGT reading for the place and time people will be active. Conditions can change with sunlight, humidity, wind, and nearby surfaces.

How guidelines fit in

A WBGT threshold (or Flag level) connects the WBGT reading to an action for a particular activity.

Swipe sideways to see every activity level →

Example Klimo Insights flag thresholds and recommendations by activity intensity.
WBGT & flag	Low intensity	Medium intensity	High intensity
Clear flagBelow 80°F	Normal activities. Maintain hydration and watch for signs of heat stress.
Green flag80–84.9°F	Regular activities.	10-minute break each hour. Hydrate.	10-minute break each hour. Hydrate.
Yellow flag85–87.9°F	10-minute break each hour. Increase hydration.	20-minute break each hour. Reduce overly strenuous activities.	30-minute break each hour. Ideally reduce any activity to 45 minutes.
Red flag88–89.9°F	15-minute break each hour. Increase hydration.	20-minute break each hour. Reduce strenuous activities.	30-minute break each hour. Ideally reduce any activity to 45 minutes.
Black flag90°F or higher	20-minute break each hour. Stay in the shade.	20 minutes of activity, then 40 minutes of rest. Ideally seek shade or indoors.	Suspend all outdoor activity. Move indoors or reschedule.

Use the policy that applies to your activity.

This table is an example. Workload, acclimatization, clothing, and health can change the appropriate response. How to use WBGT thresholds ↗

Explore WBGT policies by stateCompare high-school practice and game requirements on an interactive map.

Why it’s important

The same air temperature can mean different heat stress.

Sun, nearby radiant heat, humidity, and wind all affect the environment around an active person. WBGT brings them together; air temperature and heat index leave some of them out.

Weather factors represented by air temperature, heat index, and WBGT
	Air temp.	Heat index	WBGT
Air temperature	✓	✓	✓
Humidity	—	✓	✓
Sun & radiant heat	—	—	✓
Actual wind	—	—	✓

Heat index conditions: National Weather Service ↗

Where WBGT helps

The same reading can support different decisions in different settings.

School athleticsPlan practices and games; apply rules for equipment, rest, and activity changes. Outdoor workPlan shifts and heat controls around workload, clothing, and acclimatization. Public operationsPrepare for field activities and events as local conditions change.

Explore

Play around with air temperature, humidity, sun, and wind values to see how the estimated WBGT and heat index respond.

Turn on JavaScript to change the weather controls. The default example below shows 90°F air, 50% humidity, strong sun, and a 4 mph breeze; the rest of this guide remains available.

Hot air · moderate humidity · strong sun · a light breeze

ESTIMATED WBGT

86.9°FYellow flag ↓

ESTIMATED HEAT INDEX94.6°FAir + humidity only

This is simply an illustration, not a local forecast or a measurement.

How this estimate works +

This educational explorer adapts the Liljegren et al. (2008) heat-balance model ↗ to estimate natural wet-bulb and globe temperatures. It uses the outdoor WBGT formula under solar load and the no-solar formula at zero. It does not convert heat index into WBGT.

Fixed assumptions: sea-level pressure (1,013.25 hPa), sun 60° above the horizon, 80% direct solar radiation with sunlight, a 150 mm black globe, and a surface at air temperature with 20% reflectivity and 0.98 emissivity. Wind is at sensor height (about 2 m) with a 0.13 m/s model minimum.

Sun level: 0–100% represents 0–1,000 W/m² incident solar radiation, not cloud cover. Zero removes all solar load for an ideal-shade comparison; real shade can retain diffuse and nearby radiant heat.

Heat index: follows the National Weather Service method ↗, including Rothfusz regression, humidity adjustments, and the simpler estimate for cooler conditions. Only air temperature and humidity enter this calculation. Extreme combinations may exceed its validated range.

Flag colors: use the example guidance below and classify WBGT rounded to one decimal in °F. Changing display units leaves the flag unchanged.

Natural wet bulb77.3°F

Black globe118.8°F

Solar radiation800 W/m²

Current formula: 70% natural wet bulb + 20% globe + 10% air temperature.

This product includes software produced by UChicago Argonne, LLC under Contract No. DE-AC02-06CH11357 with the Department of Energy. Software license and attribution ↗. This educational adaptation is separate from Klimo’s forecast system.

FAQs

Practical answers about readings, thresholds, and local conditions.

Is WBGT the same as the heat index or “feels like”? +

No. Heat index uses air temperature and humidity and assumes shade and light wind. WBGT also responds to sun and other radiant heat and to actual wind. Their numbers and safety thresholds are not interchangeable.

Is WBGT the same as wet-bulb temperature? +

No. Natural wet-bulb temperature is one of the three measurements used to calculate outdoor WBGT. Under solar load, it contributes 70%; globe temperature contributes 20%, and air temperature contributes 10%. See the calculation above.

Why can WBGT be lower than the heat index? +

They use different measurements and scales. A lower numerical WBGT does not mean less danger; compare each reading only with guidance written for that index. See the National Weather Service comparison ↗.

What WBGT is too hot? +

There is no universal cutoff. The appropriate action depends on the activity’s workload, clothing or equipment, acclimatization, and governing policy. Use thresholds in the correct units for your setting. See how guidelines fit in.

How do I use a WBGT reading with a heat policy? +

Find the reading for the place and time of activity, match it to the policy’s WBGT band, and follow its instructions for timing, intensity, rest, equipment, or stopping. Recheck as conditions change; a reading alone is not an activity decision.

Can WBGT be high on a cloudy day or in shade? +

Yes. Warm, humid air and low wind can still raise heat stress when direct sun is reduced. Sun and shade can also have different WBGT values, so use the reading for the actual exposure.

Can a forecast replace an on-site WBGT meter? +

A forecast helps plan ahead, but local shade, wind, surfaces, and nearby heat sources may produce different conditions where people are active. Use an on-site measurement when your policy requires it or local conditions differ from the forecast.

Does WBGT account for heavy work or protective clothing? +

WBGT measures the environment. Hard work adds internal heat, while some clothing and protective equipment reduce heat loss. Apply the workload and clothing adjustments required by the relevant heat-safety guidance.

Does WBGT tell me my body temperature or guarantee safety? +

No. It does not measure a person’s core temperature or individual heat strain. People can respond differently to the same conditions. Respond to possible heat illness symptoms regardless of the WBGT reading and follow your emergency procedures.

Sources: National Weather Service ↗ · OSHA ↗ · NIOSH ↗

Research

Four studies on heat policy, performance, and event planning. Open a row for the study’s scope and limits.

POLICY · 2020Georgia high school football35–100% lower illness rates

Georgia’s combined WBGT, acclimatization, and practice rules were followed by lower rates of heat syncope and heat exhaustion across WBGT categories in a before-and-after study of 341,348 athlete-exposures. It evaluated the policy package, so it cannot isolate WBGT alone. The 100% reduction means no cases were observed in one category after the change.

Cooper et al. “Heat Policy Revision for Georgia High School Football Practices Based on Data-Driven Research.” Journal of Athletic Training 55(7), 673–681.

Study ↗

PRESEASON · 2019Heat policies across states55% lower illness rate

Mandatory heat-acclimatization guidance was associated with a lower exertional heat illness rate across 2.7 million preseason high school football athlete-exposures. The adjusted rate ratio was 0.45 (95% confidence interval: 0.23–0.87). This observational analysis assessed broader preseason rules, not WBGT alone; association does not prove causation.

Kerr et al. “The Association between Mandated Preseason Heat Acclimatization Guidelines and Exertional Heat Illness during Preseason High School American Football Practices.” Environmental Health Perspectives 127(4), 047003.

Study ↗

PERFORMANCE · 2022Marathons and endurance races1,258 races analyzed

Performance declined roughly 0.3–0.4% per 1°C outside optimal WBGT conditions. WBGT predicted performance better than air temperature alone in the events studied. Running and race walking had different optimal conditions and responses. The average relationship is not a fixed penalty for every event or athlete. A 1°C change equals 1.8°F.

Mantzios et al. “Effects of Weather Parameters on Endurance Running Performance: Discipline-specific Analysis of 1258 Races.” Medicine & Science in Sports & Exercise 54(1), 153–161 (published online 2021).

Study ↗

MODELING · 2025Planning match times around heat14 of 16 host locations

A historical-weather model found outdoor WBGT could exceed 28°C in 14 host locations, supporting a move away from afternoon hours for higher-risk matches. Researchers used 2003–2022 weather data for June–July heat risk at 2026 host locations. These are regional modeled conditions, not measured 2026 match or indoor stadium conditions, or a universal cancellation threshold.

Mullan et al. “Extreme heat risk and the potential implications for the scheduling of football matches at the 2026 FIFA World Cup.” International Journal of Biometeorology 69, 753–763.

Study ↗

All About Wet Bulb Globe Temperature

What is WBGT?

Four conditions shape the number

How WBGT is calculated

Natural wet bulb

Black globe

Air temperature

How to use WBGT

How guidelines fit in

Why it’s important

Where WBGT helps

Explore

FAQs

Research

Check WBGT for your location.